A double-ended pen typically has a pair of elongated liquid-holding reservoirs each having a rear end, a front end having a liquid-applying tip, and a retaining formation. An intermediate coupling sleeve centered on an axis has a pair of axially oppositely open seats each receiving a respective one of the rear ends and provided with formations for retaining the respective rear ends securely therein against axial displacement relative to the coupling sleeve. A pair of respective caps each adapted to fit over a respective one of the reservoirs each have an internal retaining formation complementary to and engaged with the retaining formation of the respective reservoir in a mounted position of the cap. The reservoirs can hold inks (see U.S. Pat. No. 3,917,416) in which case the tip is fountain, ballpoint, or felt-pen nib, or can contain cover-up or eraser liquids (see U.S. Pat. No. 4,213,717), or even different types of makeup in which case the tip can be a felt wick, a brush, a sponge or other appropriate applicator. Virtually any liquid can be dispensed by such a pen. Furthermore, the one reservoir can hold a supply of pencil leads in which case the tip can be a standard mechanical-pencil nib.
In any case it is important to be able without difficulty to take the cap off the end of the pen that will be used. It is standard, therefore, to provide screwthreads that retain the caps on the respective reservoirs, and the rear ends of the caps abut axially when they are both mounted. Thus the one end of the pen is gripped and the caps are twisted oppositely to remove one of the caps. It is, however, only a fifty-fifty chance that the right cap will unscrew. This is clearly unsatisfactory.
German utility model 7,314,439 describes a system where the caps are snap fitted to the pen. The shaft of the pen is provided with radially outwardly open grooves into which radially inwardly projecting ridges of the caps can fit. Such a system is still relatively disadvantageous since one normally needs a good grip on the pen to pull off one of the caps, and this can result in the wrong cap being pulled off. Furthermore the assembled pen does not present a smooth exterior that facilitates it being inserted into a pocket.